The present invention is related to tapping optical signals and attenuating optical signals and more particularly to equipment and methods which provide dynamic variable tapping and/or dynamic variable attenuation of optical signals in a fiber optic network.
Increased demand for data communication and growth of the internet have resulted in increased demand for communication capability within metropolitan areas. There has also been an equally large increase in demand for communication capability between large metropolitan areas. Optical communication systems formed by networks of fiber optic cables are being developed and installed to meet this increased demand.
Various types of optical switches and techniques are currently used in communication systems and computer systems. Many currently available optical switches are based upon optoelectric and electrooptic conversion of light signals and electrical signals within the associated optical switch. One type of presently available optical switch includes a matrix of thermooptic switching elements interconnected by waveguides formed on a silica substrate. Switching of light signals is accomplished by the use of thin film heaters to vary the temperature of the switching elements. Electrical circuits are also provided to supply switching current to the heaters. A heat sink may be provided to dissipate heat caused by the switching operations. One example of such switches is shown in U.S. Pat. No. 5,653,008.
Some presently available optical switches include a semiconductor substrate with vertical current flow to heat active regions of an associated optical switch. One example of such switches is shown in U.S. Pat. No. 5,173,956. Some optical switches require mode perturbation to generate required mode patterns for the desired switching function. Examples of such optical switches include directional couplers and Mach Zhender interferometers. Such optical switch designs often have poor scalability, relatively high manufacturing costs and low optical signal bandwidth.
Various types of optical signal amplifiers, wavelength division demultiplexers, optical switches, wavelength division multiplexers and techniques are currently used in optical communication systems. Optical signal amplifiers, wavelength division multiplexers and demultiplexers and other components associated with optical communication systems typically function best when respective signal levels of associated optical signals are substantially equal with each other. A variation in signal level of multiple wavelength optical signals may result in an undesirable signal to noise ratio and resulting poor performance by switches, amplifiers and other optical components.
Multiple wavelength optical signals are often collectively amplified by a light amplifier. The amplification factor of many light amplifiers is dependent upon the wavelength of each optical signal. Therefore, amplification factors for multiple wavelength optical signals often vary depending on the specific wavelength of each signal. The resulting difference between signal levels of respective multiple wavelength optical signals amplified by a single amplifier is often relatively small. However, when a large number of light amplifiers (ten or more) are used in a fiber optic communication system, the variation in signal levels becomes cumulative and may result in unsatisfactory lowering of associated signal to noise ratios. Therefore, variable optical attenuators are often provided at the input stage and/or output stage of light amplifiers in both large metropolitan communication systems and long distance fiber optic communication systems to adjust signal levels of multiple wavelength light signals to maintain desired signal to noise ratios.
Variable optical attenuators are often included in optical communication systems to maintain a desired signal level for each optical signal or wavelength. Examples of variable optical attenuators (VOA) include natural density filters which are often used to suppress the amount of light depending on wavelength characteristics. Other variable optical attenuators include mechanical devices which position a glass substrate so that light signals may be attenuated by varying the position of the glass substrate. Still other variable optical attenuators attenuate light signals by rotating the polarization of each light signal as it passes through a Faraday element.
Optical taps are also included in many optical communication systems to monitor both performance of individual components and performance of the overall system.
In accordance with teachings of the present invention, a system and method are provided to attenuate optical signals and/or tap optical signals and to substantially reduce or eliminate disadvantages and problems associated with presently available optical systems. The present invention provides a dynamic variable optical attenuator and a dynamic variable optical tap which use substantially total internal reflection (TIR) at junctions formed by two waveguides intersecting with each other at a selected angle. For some applications, the desired internal reflection will occur at a junction between the two waveguides in response to heating from a thin film electrode. For other applications, the desired internal reflection may be produced by electrooptic, magnetooptic or acoustooptic effects. A dynamic variable optical attenuator and dynamic variable optical tap formed in accordance of teaching of the present invention are satisfactory for use with optical systems that communicate at very high data-rates between terminals.
Technical advantages of the present invention include a low cost, reliable optical attenuator and/or optical tap which may be formed as separate, individual components or may be integrated with other components, such as optical switches. An optical attenuator or tap incorporating teachings of the present invention is intrinsically a wide band device covering all S-band, C-band and L-band optical signals. The optical attenuator or optical tap may be fabricated on a wide variety of materials such as polymer/SiO2, polymer/polymer, polymer/polymer/polymer and semi-insulating/semiconductor substrates. The optical attenuator or tap may be used in general purpose optical communication systems including fiber optic networks associated with modern metropolitan communication systems.
One aspect of the present invention includes a dynamic, variable optical tap having at least a two channel waveguide array. Each junction formed by each intersection of the waveguides is relatively small which substantially eliminates cross talk between the respective waveguides while maintaining a relatively large dynamic range of low-cross talk. Therefore, the optical tap has a relatively small insertion loss. Beam propagation methods (BPM) may be used to determine characteristics, such as dynamic range, of each junction. A respective electrode may be disposed on each junction or intersection of the waveguides along with an electrical current input port. Current may be introduced from the input port to flow through the electrode to the grounding port to create sufficient heat by the electrode to modulate the index of refraction within portions of the waveguides disposed adjacent to the electrode. As a result of heat created by the electrode, the index of refraction may be reduced. The portion of the junction disposed adjacent to the electrode may encounter total internal reflection which provides desired taping of optical signals communicated through one of the waveguides. The location of the electrode may be selected in accordance with teachings of the present invention to maximize optical efficiency of the tap.
Another aspect of the present invention includes a dynamic, variable optical attenuator having at least a two channel waveguide array. Each junction formed by each intersection of the waveguides is relatively small which substantially eliminates cross talk between the respective waveguides while maintaining a relatively large dynamic range of low-cross talk. Therefore, the optical attenuator has a relatively small insertion loss. Beam propagation methods (BPM) may be used to determine characteristics, such as dynamic range, of each junction. A respective electrode may be disposed on each junction or intersection of the waveguides along with an electrical current input port and a grounding port. Current may be introduced from the input port to flow through the electrode to the grounding port to create sufficient heat by the electrode to modulate the index of refraction within portions of the waveguides disposed adjacent to the electrode. As a result of heat created by the electrode, the index of refraction may be reduced. The portion of the junction disposed adjacent to the electrode may encounter total internal reflection which provides desired attenuation of optical signals communicated through one of the waveguides. The location of the electrode may be selected in accordance with teachings of the present invention to maximize switching efficiency of the attenuator. For some applications an optical tap may be formed in accordance with teachings of the present invention on at least one of the waveguides associated with each optical attenuator.
Two dimensional arrays formed in accordance with teachings of the present invention may be satisfactorily integrated to form a wide variety of arrays such as two by two, eight by eight, sixteen by sixteen and sixty-four by sixty-four. The resulting arrays may be hermetically sealed using appropriate semiconductor fabrication techniques. An optical attenuator and/or tap formed in accordance with teachings of the present invention may be satisfactorily used in optical communication systems including fiber optic networks having cable lengths ranging from one hundred meters to thousands of kilometers.
Typical specifications for an optical attenuator and/or tap device formed in accordance with teachings of the present invention include cross talk between adjacent waveguides of less than thirty (30) dB, insertion loss of less than five (5) dB per waveguide, polarization independent return loss greater than forty (40) dB and relatively fast response time for a thermal optic device. For various applications the response time may range between less than twenty milliseconds to much less than a millisecond.
The quality of a light signal is generally determined by the ratio between the signal level and the intensity of noise associated with the signal level. This ratio is commonly referred to as the signal-to-noise ratio (SNR). Dynamic variable optical attenuators formed in accordance with teachings of the present invention may be satisfactorily used to adjust the intensity or signal level of multiple wavelength light signals communicated through an optical communication system to establish a desired signal-to-noise ratio for optimum performance of amplifiers, wavelength division multiplexers and demultiplexers and other components associated with optical communication systems.